1. Field of the Invention
The present invention concerns new organic opto-electronic devices, such as organic light emitting diodes (OLEDs), organic displays, organic solar cells, photodiodes and the like. Also addressed is a new method for making such devices.
2. Discussion of the Related Art
Organic light emitting diodes (OLEDs) are an emerging technology with potential applications as discrete light emitting devices, or as the active elements of light emitting arrays, such as flat-panel displays. OLEDs are devices in which a stack of organic layers is sandwiched between two electrodes. At least one of these electrodes must be transparent in order for light --which is generated in the active region of the organic stack--to escape. To achieve high efficiency and low voltage operation each of the organic layers as well as the electrodes have to be optimized for their individual function; charge carrier injection, charge carrier transport, charge carrier recombination, and light extraction. Despite the great progress achieved in recent years, full optimization is difficult to obtain using conventional approaches, as will be outlined below.
OLEDs emit light which is generated by injection electroluminescence (EL). Organic EL at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al., Journal Chem. Phys., Vol. 38, 1963, pp. 2024, and in "Recombination Radiation in Anthracene Crystals", Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred largely by two reports of high efficiency organic EL. These are C. W. Tang et al., "Organic electroluminescent diodes", Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and by a group from Cambridge University in Burroughes et al., Nature, Vol. 347, 1990, pp. 539. Tang used vacuum deposition of molecular compounds to form OLEDs with two organic layers. Burroughes spin coated a polymer, poly(p-phenylenevinylene), to form a single-organic-layer OLED. The advances described by Tang and in subsequent work by N. Greenham et al., Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in OLED design derived from the selection of appropriate organic multilayers and electrode metals.
To date, virtually all OLED device structures have been built on glass substrates coated with indium-tin oxide (ITO), which serves as a transparent anode, i.e. light is emitted through the anode and substrate. This kind of a device structure is usually referred to as cathode-up structure. The cathode is typically a low-workfunction elemental metal or low-workfunction alloy, e.g. Ca, Al, Mg/Ag, or Al/Li. Such cathodes are opaque. These low-workfunction elemental metals and alloys belong to the first class of cathode materials considered for OLEDs.
In order to enable a variety of possible applications, OLED structures suitable for opaque substrates (i.e. substrates other than the conventional glass substrates) are highly desirable. For example, if OLEDs could be fabricated on silicon, this would permit the use of an integrated active-matrix drive scheme. In such a structure light must be emitted through the uppermost layers of the device rather than through the substrate. One possible solution would be to build OLEDs by depositing the layers in the opposite order, which means a structure would be obtained with the transparent ITO anode deposited on top (referred to as anode-up structure). This has proved difficult, presumably due to the harsh conditions under which the ITO is deposited.
Alternatively, devices could be fabricated with the normal sequence of layers provided that a transparent cathode could be found. Gallium Nitride (GaN) has already been suggested as one possible cathode material for these kind of alternative cathode-up structures, as disclosed and described in an international patent application WO98/07202 with title "Gallium Nitride Based Cathodes for Organic Electroluminescent Devices and Displays". The international publication date of this patent application is Feb. 19, 1998. The GaN is a non-degenerate, wide-bandgap semiconductor (nd-WBS). As described in the international application, all nd-WBSs have the advantage that their wide bandgap makes them transparent. It has been shown that the wide bandgap also leads to a favorable alignment of either the conduction band or valance band with the lowest unoccupied molecular orbitals (LUMO) or highest occupied molecular orbital (HOMO) of the organic material into which charge is to be injected. These non-degenerate, wide-bandgap semiconductors form a second class of cathode materials considered for OLEDs.
It has been shown that improved performance can be achieved when the electrode materials are chosen to match the respective molecular orbitals of the organic material into which it is supposed to inject carriers. By choosing the optimized electrode materials the energy barriers to injection of carriers can be reduced.
It has been shown in U.S. Pat. No. 5,340,619, with title "Method of manufacturing a color filter array", that ink-jet printing or other printing technologies can be used to coat a substrate. First, the substrate is coated with a blue resin which is baked (cured) before red and green colored polyimide dyes are each added and cured respectively. After all the colors are added and cured, laser ablation is used to reduce the thickness of the coating to develop a color filter array.
With multilayer device architectures now well understood and commonly used, the major performance limitation of OLEDs is the lack of ideal contact electrodes, and in particular the lack of transparent and conducting materials which can be deposited on organic layers without causing damage having a detrimental effect on the device performance and reliability.
One figure of merit for electrode materials is the position of the energy levels (bands) relative to those of the organic materials. In some applications it is also desirable for the electrode material to be transparent, as mentioned above. Furthermore, the electrode should be chemically inert and capable of forming a dense uniform film to effectively encapsulate the OLED. It is also desirable that the electrode and/or electrode deposition does not lead to a strong quenching of EL.
Another important figure of merit for electrode materials is the ease of handling and problem-free deposition on organic layers. Futhermore, the electrode materials have to be compatible to the organic materials underneath which is often difficult to achieve.
The incompatibility problems inherent to most electrode materials used so far can be extended and generalized. The most severe limitations in the deposition of metals and semiconductor-based electrodes onto organic layers are:
damage of the organic materials during the deposition which often leads to irreversible changes within the organic layers and at their interfaces; PA0 damage of the organic materials due to heat treatments required to obtain electrodes with good physical, mechanical and electrical and electro-optical properties. High process temperatures lead to thermal damage of the organic materials such as crystallization, interdiffusion and intermixing of the organics. PA0 low manufacturing yield because the more processing steps are performed, the lower the output of fully functional devices gets. PA0 reduced number of materials available, because not-only electrical, but also chemical compatibility with the organic materials is required. For example, up to now no polymers can be deposited on top of evaporated organic layers, because of dissolving problems. PA0 the size and shape of the spacers have to be such that sufficient electrical contact is provided between the organic layers on the different pieces or the organic layer and the electrode on the different parts. PA0 the spacers must prevent short circuits. This means that the spacers consist either of non conducting materials or if they are conducting they should be electrical isolated at least from one electrode structure. PA0 the spacers have to be rigid, to protect the organic layers from damage. This is especially important for flexible device structures. PA0 the total thickness of the spacers have to be chosen in such a way, that sufficient electrical contact between the organic layers is provided and damage between the organic layers is avoided. PA0 electrodes can be deposited at conditions otherwise not suited for OLED formation (e.g. high temperature, aggressive chemical environment, ion damage (sputter damage), high energy particle processes); PA0 electrodes can be easily patterened; PA0 separate testing and inspection of both `halfs` is possible. This helps to increase the yield and thus reduces manufacturing costs. PA0 each `half` can be made of optimized materials and using optimized processes without having to take care of incompatibility issues. PA0 If appropriately designed, the spacers also serve as studs or posts which protect the sensitive parts from being mechanically damaged during handling, as will be discussed in connection with FIGS. 1A and 1B. PA0 The present approach is well suited for the formation of large area displays, for example, where the yield is one of the major cost factors. The larger an active matrix display gets, the more likely it is that one transistor fails. In such a case the whole display is not suited for use and has to be discarded. The present approach leads to a drastically increased yield which in turn allows to make cheaper display. PA0 The spacers can provide conductive connection between circuitry on the two halves. This is especially important for large area displays where the conductivity of the common electrode limits the device performance. PA0 Additional spacers can be employed that provide a conductive connection between circuitry on the two `halfs`.
So far, there is a costly and time-consuming search for better suited materials which may serve as stable, possibly transparent electrodes.